1. Field of the Invention
The invention relates to an image sensor, and more particularly, to a pixel structure of a complementary metal oxide semiconductor (CMOS) image sensor.
2. Description of the Related Art
Generally, an image sensor captures images by using a doped semiconductor element having a characteristic that responds to some kind of an external electromagnetic energy (for example, light). Light emitted from each object in the natural world has a characteristic energy value such as a wavelength, etc. A pixel of the image sensor senses the light emitted from the object and converts a light energy value to an electrical energy value. That is, the pixel of the image sensor generates the electrical value in response to the light energy, etc., that is emitted from the object.
FIG. 1 represents a cross section of a photo-diode according to a conventional 3-transistor CMOS active pixel, with related circuitry. According to the conventional 3-transistor CMOS active pixel, an N+-type dopant layer 11 forming a side junction of the photo-diode is in contact with an N+-type floating diffusion layer 13. Therefore capacitance of the photo-diode is substantially equal to the sum of capacitances respectively provided by the N+-type dopant layer 11 and the N+-type floating diffusion layer 13. Thus, an image sensor using a conventional 3-transistor CMOS active pixel has poor sensitivity. To overcome this disadvantage of the 3-transistor CMOS active pixel, a 4-transistor CMOS active pixel has been developed.
FIG. 2 illustrates a cross section of a photo-diode according to a conventional 4-transistor CMOS active pixel, with related circuitry. In a conventional 4-transistor CMOS active pixel, a transfer transistor 35 is used to remove noise. The transfer transistor 35 is controlled by a transfer control signal TX. An N+-type dopant layer 21, forming a side junction, is separated from an N+-type floating diffusion layer 23. As a result, the conventional 4-transistor CMOS active pixel provides an image sensor with high sensitivity and high quality. However, the conventional 4-transistor CMOS active pixel has a reduced light receiving area due to the additional element, i.e., the transfer transistor 35.
In summary, both types of conventional CMOS active pixels have problems: The conventional 3-transistor CMOS active pixel has poor sensitivity. The conventional 4-transistor CMOS active pixel has a reduced light receiving area.
It is an object of the invention to solve the above-described problems. In particular, it is an object of the invention to provide a CMOS active pixel capable of high sensitivity while minimizing a reduction in light receiving area.
To achieve the above object, there is provided a CMOS active pixel formed on a semiconductor substrate. According to one aspect of the invention, the CMOS active pixel includes a floating diffusion layer, a photo-diode, a reset circuit and an output circuit. The floating diffusion layer is doped with a dopant of a first dopant type and receives a signal charge. The photo-diode generates the signal charge, and transfers the signal charge to the floating diffusion layer. The photo-diode has a lower diode dopant layer of the first dopant type, and an upper diode dopant layer of a second dopant type. The polarity of the dopant of the second dopant type is opposite to that of the first dopant type. The upper diode dopant layer is formed on the lower diode dopant layer. The reset circuit, in response to a control signal, controls the voltage of the floating diffusion layer to a reset voltage level. The output circuit generates an output signal corresponding to the voltage level of the floating diffusion layer. In this case, the electric potential energy of the lower diode dopant layer is higher than that of the floating diffusion layer, in an initial state of the pixel. Preferably, a doping concentration of the lower diode dopant layer is less than that of the floating diffusion layer.
According to another aspect of the invention, a CMOS active pixel formed on a semiconductor substrate includes a floating diffusion layer, a photo-diode, a reset circuit and an output circuit. The floating diffusion layer is doped with a dopant of a first dopant type and receives a signal charge. The photo-diode generates a signal charge and transfers the signal charge to the floating diffusion layer. The photo-diode has a first and a second lower diode dopant layer each of the first dopant type and an upper diode dopant layer of a second dopant type. The polarity of the dopant of the second dopant type is opposite to that of the first dopant type. The upper diode dopant layer is formed on the first and the second lower diode dopant layers. The first lower diode dopant layer is formed between the floating diffusion layer and the second lower dopant layer. The reset circuit controls a voltage of the floating diffusion layer to a reset voltage level in response to a control signal. The output circuit generates an output signal corresponding to the voltage level of the floating diffusion layer. In this case, an electric potential energy of the first lower diode dopant layer in an initial state of the pixel is higher than that of the floating diffusion layer. An electric potential energy of the second lower diode dopant layer in the initial state of the pixel is higher than that of the first lower diode dopant layer. Preferably, a doping concentration of the first lower diode dopant layer is less than that of the floating diffusion layer. Further, a doping concentration of the second lower diode dopant layer is less than that of the first lower diode dopant layer.
According to still another aspect of the invention, CMOS includes a floating diffusion layer, a photo-diode, a reset circuit and an output circuit. The floating diffusion layer is a first dopant type and receives a signal charge. The photo-diode generates the signal charge according to an energy inputted thereto, and transfers the signal charge to the floating diffusion layer. The photo-diode has a lower diode dopant layer of the first dopant type, an upper diode dopant layer of a second dopant type and a separating layer. The polarity of the upper diode dopant layer is opposite to that of the lower diode dopant layer. The upper diode dopant layer is formed on the lower diode dopant layer. The separating layer is formed between the lower diode dopant layer and the floating diffusion layer. The reset circuit controls a voltage of the floating diffusion layer to a reset voltage level in response to a control signal. The output circuit generates an output signal corresponding to the voltage level of the floating diffusion layer. In this case, the electric potential energy of the separating layer in an initial state of the pixel is higher than that of the floating diffusion layer.